Table of Contents

toc Guiding Questions: Electric Charge, Forces, and Fields

9/7/11

9/8/11

Sticky Tape Lab 9/12/11

9/13/11

9/14/11

9/15/11

9/21/11

Guiding Questions: Electric Charge, Forces, and Fields

1. What is the structure and properties of an atom?

• The atom is made up of protons, neutrons, and electrons
• The atom has protons and neutrons that make up most of the atom
• Protons and neutrons are located at the center of the atom
• The elections travel around the center
• The proton is positively charged, electron is negatively charged, and the neutron does not have a charge
• An atom has no charge

1. What is the symbol and unit of electric charge?

• Symbol is q or Q
• The unit is C (Coulomb)

1. Distinguish between positive and negative charges in as many ways as possible.

• Less electrons than protons is positively charged
• Less protons than electrons is negatively charged
• negative charges are electrons
• positive charges are ions
• ions are bigger in size than electrons
• mass of a ion is greater than electrons
• electrons can flow in orbital clouds and mobile
• positive charges are immobile

1. Describe the properties of electric forces.

• The properties of electric force is a non-contact force
• An object can exert this force upon other objects between both charged and uncharged objects

1. Distinguish between insulators and conductors.

• Conductors are allow electrons to move freely between two things
• Insulators hold electrons in an object.

1. What is polarization?

• When two objects separate into opposites

1. How does a neutral object acquire charge?

• It is induced
• It is placed next to a charged object near the conducting object

1. Distinguish between the 3 charging processes.

• Friction – two objects rub together
• Induction – without actually touching any other object
• Conduction – contact of a charged object to an uncharged object

1. What is the law of electric charge?

10.What is an electric field?
It is the area that surrounds electrically charged objects.

11.What are the characteristics and properties of an electric field? See applet: http://www.gel.ulaval.ca/~mbusque/elec/main_e.html

12.What are the “players” involved in an electric field?

13.What are electric field lines?
The direction that positive atoms would move

14.What are 4 characteristics of electric field lines?
Can not intersect each other, out of the positive and into the negative, the more lines the stronger the E, one to surface

15.Go to http://www.physicsclassroom.com/Class/estatics/u8l4c.cfm. Scroll to the bottom of the page and do the “Check Your Understanding” questions.

1. How is electric force computed using the superposition principle?
2. What are the properties of an isolated conductor?

18.Explain how lightning occurs and lightning rods work. Use any reference you would like, but be sure to note down the bibliographical information.

1. Summarize the Millikan Oil Drop Experiment, using a labeled diagram. See applet: http://www.youtube.com/watch?v=XMfYHag7Liw
2. Describe how a Van De Graaff generator works.

Practice Questions

1. What is the charge on a rod that has 15 excess electrons?

15e X (1.6x10^-19C) / (1e) = - 2.4x10^-18C

1. What is the charge on a pith ball that has 3.15 x 1016 electrons?

3.15x10^16e- X (-1.6x10^-19C) / (1e-) = -0.000504C OR -5.04X10^-3

1. How many electrons are missing from a balloon that has a charge of 4.19 x 10-5 C?

4.19X10^-5 X (1e-) / (1.6x10^-19C) = 2.62x10^14e-

1. If a metal object receives a positive charge, does its mass increase, decrease, or stay the same? What happens to its mass if the object receives a negative charge?

If it is positive it loses electrons it makes it smaller. If it turns negative it increases because ti is gaining electrons.

1. A 4.5 x 10-9 C charge is located 3.2 m from a –2.8 x 10-9 C charge. Find the electrostatic force exerted by one charge on the other.
2. What is the magnitude and direction of the force between two identical 10 microcoulomb charges 0.015-m apart?
3. Consider the electric force between a pair of charged particles a certain distance apart. By Coulomb's law:What does the direction of electric field lines show?
   1. If the charge on one of the particles is doubled, the force is _.
   2. If, instead, the charge on both particles is doubled, the force is _.
   3. If instead the distance between the particles is halved, the force is _.
   4. If the distance is halved, and the charge of both particles is doubled, the force is ___ as great.
4. Can you describe what charges are present in each of the diagrams shown here?
5. A proton accelerates from rest in a uniform electric field of 640 N/C. Find the magnitude of the acceleration of the proton.
6. Suppose you touch a negatively charged object to a metal sphere so that electrons move from the charged object onto the sphere. After far separation of the charged object, how will the electrons on the sphere be distributed?
7. If two negative charges are held close together, how will the charges move when released?
8. Three charges are arranged as shown below. Find the magnitude and direction of the electrostatic force on the charge at the origin.

1. Find the electric field in the empty corner as shown in problem 13.

15.Describe the electric field strength at the six labeled locations of this irregularly shaped, charged object. Use the phrases "zero," "relatively weak," "moderate," and relatively strong" as your descriptions.

16.A Van de Graaf generator is charged so that the electric field at its surface is 2.0 x 104 N/C. Find the electric force exerted on a proton released at its surface and the acceleration of the proton at this instant of time. Two small silver spheres, each with a mass of 100-g, are separated by 1.00 m. Calculate the fraction of the electrons in one sphere that must be transferred to the other in order to produce an attractive force of 1.00 x 104 N (about a ton) between the spheres.

1. Relate these ideas to each other. Try to make links from each of the 5 concepts to all of the others.

9/7/11

9/8/11

Summary: Method 2a

1. A) The structure of the atom. An atom has a three parts to it electrons, protons, and neutrons. The nucleus of an atom is the center of the atom. The nucleus is made up of neutrons and protons and is very dense. Electrons travel around the neutron. The proton is positively charged and electrons are negatively charged, while neutrons have no charge. An atom has no charge.

B) The differences between positive and negative charges. Positive charges are made when there are less electrons than protons. Positive charges are called ions, ions are bigger than electrons, and the mass is greater. Negative charges are called electrons.

1. I was confused how charge interactions worked. I thought that when a charged object interacted with a neutral object whatever the charge whether it be positive or negative; if it was a positive and neutral it would attract and the opposite. Now I understand that whatever the charge of the object it will attract the neutral object.
2. What does polarization actually do to the object?
3. Polarization

Sticky Tape Lab 9/12/11

OBSERVATIONS:

#5: Sketch with labeled force vectors for two top tapes close enough to affect each other. Repel T1 ß à T2	Sketch with labeled force vectors for two top tapes half as far apart as left sketch Nothing, probably repelled T1 ß à T2
#8: Describe paper on paper interaction Nothing gappens	#9: Describe foil on foil interaction Nothing happens
#13: Describe top tape and foil interaction Attract Diagram with forces T2àß F	Describe top tape and paper interaction Attract Diagram with forces T2 àß P
#13: Describe top tape and top tape interaction Repelled Diagram with forces T1 ßà T2	Describe top tape and bottom tape interaction Attract Diagram with forces T2àß B1
#13: Describe bottom tape and foil interaction Attract Diagram with forces B2àß F	Describe bottom tape and paper interaction Attract Diagram with forces B2àß P

#13: Describe bottom tape and top tape interaction Attract Diagram with forces B2àß T1	Describe bottom tape and bottom tape interaction Repel Diagram with forces B2 ßà B1
#14: Describe PVC rod and paper interaction Attract, medium PVCàß P	Describe PVC rod and foil interaction Attract, strong PVCàß F
#14: Describe PVC rod and top tape interaction Attract, least attracted PVCàß TT	Describe PVC rod and bottom tape interaction Attract, small PVCàß TB
#15: Describe Lucite rod and paper interaction Attract Làß P	Describe Lucite rod and foil interaction Attract Làß F
#15: Describe Lucite rod and top tape interaction Repel L ßà TT	Describe Lucite rod and bottom tape interaction Attract Làß BT
#16: State top and top tape interaction based on charge Repel TT ßà TT	State top and bottom tape interaction based on charge Attract T tàß BT
#16: State bottom and bottom tape interaction based on charge Repel BT ßà BT

Discussion Questions:

1. Explain how materials become charged through their interaction with one another.

Material became charged through interactions through friction, conduction, and induction. The neutral charged objects interacted with each other give them either a negative, positive, or neutral charge.

1. Why, when you stroke a cat's fur, or comb your hair on a cold, dry day can you hear a crackling sound? Doing these things in a darkened room, you can actually see sparks. Explain.

You can hear this because the object that is crackling is being charged through friction and conduction. Whatever object is being charged electrons are jumping between which is charge and on a cold, dry day electrons can jump between atoms much easier because nothing is in their way.

1. Photocopying machines use the principles of electric charges. Do research to find out how photocopying machines work. Be sure to list your sources.

http://www.pa.msu.edu/sciencet/ask_st/042893.html

1. Materials have a characteristic which evaluates their attraction for electrons. The Triboelectric Series orders materials by their affinity for gathering electrons through contact from other materials. The materials toward the top of the list are likely to give up electrons in these interactions whereas those at the bottom are more likely to gain electrons. Five materials are ranked as follows, with more positives meaning least desiring electrons.

PVC(++), Wool (+++++), Styrene (++++), Teflon (+), Polyester (+++)

1. Rank the materials on the scale below:

Teflon PVC Polyester Styrene Wool

Accept<--------------------------------------------------------------------------------------------------------------->Give

1. Determine the net charge on each item when the following pairs of materials are rubbed together. (In other words, which ends up giving up electrons and which ends up accepting them?)
1. PVC and Wool

Wool gives, PVC accepts

1. PVC and Teflon

PVC gives, Teflon accepts

1. PVC and Polyester

Polyester gives, PVC accepts

1. Teflon and Polyester

Polyester gives, Teflon accepts

1. Styrene and Wool

Wool gives, Styrene accepts

9/13/11

Electrostatic Forces
+q1 =q2

Fe = (ke) | q1| | q2| / d^2

q - C
d - m
Fe - N

• Field forces
• Act through space
• Electro static force is relatively strong
• Electro static force attract and repel

+q ------> <--------- -q
<---- a } increase speed
<---- v

-q <--------- ---------> -q
-----> a } increase speed
-----> v

9/14/11

9/15/11

Electric Field

• a region around a charge where another charge will feel and electric force
• a region where the other charge feels the source
• Can not intersect each other, out of the positive and into the negative, the more lines the stronger the E, one to surface

Q (source of field) q (experiencing charge)

E = Fe / q
=
E = ke x Q / d^(2)

9/21/11